Anti-Clotting Agents Explained

Updated:May 29,2014

Excerpted and adapted from "Clot? Not!," Stroke Connection Magazine, July/August 2003 (Last science update April 2014)

Clot? Not!

Antiplatelet and anticoagulant therapies are at the heart of preventing recurrent strokes. Although neither antiplatelet nor anticoagulant drugs can break up a clot (that’s a job for tPA and other clot busters being tested), both types of drugs are effective in keeping a clot from forming or stopping the growth of one. A lot of antiplatelets and anticoagulants are available to stroke survivors, and it helps to understand them.

Antiplatelets

Blood platelets are actually fragments of cells – meaning they don’t contain all the necessary cellular equipment. When a person gets a cut or scratch, platelets release thromboxane, a chemical that signals other platelets to “help out.” Without the release of thromboxane, the platelets won’t come (stick) together, no clot will form, and the cut will continue to bleed. If you have a wound, thromboxane is an indispensable self-sealing material; but if you’re a stroke survivor, thromboxane’s ability to round up “help” to form a blood clot becomes potentially life-threatening.

Antiplatelet agents, including aspirin, clopidogrel, dipyridamole and ticlopidine, work by inhibiting the production of thromboxane. Aspirin is highly recommended for preventing a first stroke, but it and other antiplatelets also have an important role in preventing recurrent strokes.

According to a statement by the American Heart Association, taking aspirin within two days of an ischemic stroke reduces the severity of the stroke. In some cases, it prevents death. For long-term (meaning for the rest of your life unless otherwise specified by your physician) prevention, antiplatelet therapy is recommended primarily for people who have had a transient ischemic attack (TIA or “mini” stroke) or acute ischemic stroke.

Despite the potential benefits, antiplatelet therapy is not for everyone. People with a history of liver or kidney disease, gastrointestinal disease or peptic ulcers, high blood pressure, bleeding disorders or asthma may not be able to take aspirin or may require special dosage adjustments.

Anticoagulants

While antiplatelets keep clots from forming by inhibiting the production of thromboxane, anticoagulants target clotting factors, which are other agents that are crucial to the blood-clotting process. Clotting factors are proteins made in the liver. These proteins can’t be created in the liver without Vitamin K – a common vitamin found in cabbage, cauliflower, spinach and other leafy green vegetables. Anticoagulants, such as warfarin (Coumadin) and heparin, slow clot formation by competing with Vitamin K. This inhibits the circulation of certain clotting factors with the exotic names of factors II, VII, IX and X. Within the past 3 years, new anticoagulants have been approved by the FDA: dabigitran, apixaban, and rivaroxaban. All of these are simpler to use and less risky than warfarin (bleeding risk is lower). 

The most important and most effective thing a survivor can do is take their health into their own hands.

Anticoagulants are considered more aggressive drugs than antiplatelets. They are recommended primarily for people with a high risk of stroke and people with atrial fibrillation. More than 2.7 million Americans have atrial fibrillation (AF), an-arrhythmia of the heart where the atria (the heart’s pumping chambers) quiver instead of beat. As a result, not all of the blood is pumped out of the heart, allowing pools to collect in the heart chamber, where clots may form.

An embolic stroke is a type of ischemic stroke that occurs when a piece of an atrial blood clot (embolus) is pumped out of the heart, circulates to the brain and becomes lodged in an artery. The American Heart Association recommends that most AF patients over age 65 receive some sort of anticoagulant therapy.

Although anticoagulants tend to be more effective for AF patients, they are generally recommended only for patients with strokes caused by clots originating in the heart. Anticoagulants tend to be more expensive and carry a higher risk of serious side effects, including bruising and skin rash and bleeding in the brain, stomach and intestines.

When used as directed, however, anticoagulants have proven very effective for AF patients. Although the potential risks seem severe, the life-saving effects give these drugs a bright upside.

Other people who may benefit from anticoagulant therapy for stroke prevention are those with blood that clots easily, and in some cases, patients with intracranial artery blockages that surgery can’t remedy.

In comparison to antiplatelets, anticoagulants tend to be affected more by other drugs, vitamins and even certain foods, making anticoagulant therapy somewhat troublesome for stroke survivors. Because warfarin competes with Vitamin K, patients taking it should consult their doctors about possible dietary restrictions, as even some vegetables might cause an imbalance if eaten in excess. See the "Potential Confusion" sidebar (opens in new window) for more information about Vitamin K. Many prescription drugs make warfarin either stronger or weaker.

Anticoagulant therapy with warfarin also requires regular blood tests to ensure the correct drug dose. A weak dosage increases the risk of stroke and heart attack, but too much puts someone at risk for bleeding. Generic brands may not be the same strength as the one prescribed by your doctor.

Read our Patient's Guide To Taking Warfarin for more information.

Combinations

Given the many benefits of antiplatelet and anticoagulant therapies, it seems logical that a combination of the two might magnify the positive effects. However, researchers have found that a combination of low-dose warfarin and low-dose aspirin is no more effective than aspirin by itself. Furthermore, in the study group, major bleeding episodes (primarily gastrointestinal) occurred nearly twice as often in the combination-therapy patients compared with the aspirin-only patients.

Because of its low cost, availability and effectiveness, aspirin is the most prescribed and used drug in antiplatelet therapy. Currently, aspirin plus extended-release dipyridamole (an antiplatelet) is the only FDA-approved combination therapy for preventing recurrent stroke.

The End of the Prescription

Anticoagulants and antiplatelets shouldn’t be thought of in terms of one therapy being superior to the other. Because all strokes and stroke survivors are unique, secondary prevention must be tailored to fit each survivor’s needs.

With so many elements factoring into treatments, confusion is inevitable, but this doesn’t have to produce helplessness. While antiplatelet and anticoagulant therapies are an important aspect of secondary prevention, they aren’t enough. The most important and most effective thing a survivor can do is take their health into their own hands. This means eating a healthy diet, exercising daily, abstaining from smoking, taking prescribed medicines according to doctor’s orders and, of course, staying educated.



This content was last reviewed on 04/30/2014.